This invention relates to an arc chute for an electric circuit breaker and, more particularly, relates to an arc chute of a refractory inorganic material having surface portions of an amorphous fused material, a major proportion of which is silica. The invention also relates to a method of making such an arc chute.
In U.S. Pat. No. 3,735,074--Frind, Korte, and Zlupko it is pointed out that exceptional current-interrupting performance can be obtained with an arc chute of a refractory inorganic material having a surface of an amorphous, fused material principally of silica. In U.S. Pat. No. 3,814,620--Bailey et al. there is disclosed a method for producing such a surface which involves arc-plasma spraying molten silica particles onto a substrate of refractory inorganic material. While such spraying can be performed relatively inexpensively, some expense and considerable care are required for its proper performance; and it would be highly desirable if the amorphous, fused principally-silica surface portion could be obtained without the need for such spraying.
Accordingly, an object of our invention is to provide an arc chute material that comprises a base of refractory inorganic material and a surface portion of an amorphous fused material, a major proportion of which is silica, produced by a process which is less expensive and easier to practice than arc-plasma spraying.
The sprayed-on, fused principally-silica coating of the above patents has exceptional durability even when repetitively exposed to high current arcs. But if for some reason this coating should be burned off or otherwise removed from a particular region of the arc chute, then the underlying substrate in this region will thereafter be directly exposed to arcs; and if the substrate is of the conventional material typically used heretofore, there will no longer be a surface principally of silica in this region to impart its desired arc-interrupting properties.
Accordingly, another object of our invention is to provide an arc chute material, initially surfaced with fused principally silica material, that is capable of reacting with an electric arc to produce a replacement surface of a fused principally-silica material if, when, and where the initial surface is removed from any portion of the arc chute.
A material that is especially suited for serving as a base for the fused principally-silica coating of the above-cited patents is phospho-asbestos material made from a mixture of chrysotile asbestos fibres, orthophosphoric acid, and a filler of zircon. The properties of this material, without the zircon filler, are described in detail in U.S. Pat. No. 2,366,485--Brink and Arone, assigned to the assignee of the present invention. The zircon filler, which is typically added in an amount of about 50 percent by weight of the mixture, improves the properties of the material by imparting added mechanical strength and refractoriness, as well as reducing the gassiness of the material when exposed to an arc. This zircon-filled phospho-asbestos material has been widely used for arc chutes, both with a fused silica coating and without such coating. Zircon-filled phospho-asbestos material with lower percentages of zircon filler has also been used, but this material has shown significantly inferior current-interrupting ability as compared to the aforesaid 50 percent zircon material.
A disadvantage of the zircon-filled phospho-asbestos material, especially that of the aforesaid 50% zircon material, is that it is quite expensive and quite heavy, due in large measure to the relatively high cost and the relatively high specific gravity of zircon.
Another object of our invention is to provide a phospho-asbestos material containing an inexpensive and relatively light-weight filler, which material when used in a circuit-breaker arc chute is capable of providing current-interrupting performance at least substantially equal to that of zircon-filled phospho-asbestos material made from a mixture containing 50 percent zircon by weight.
Another object is to provide a high quality phospho-asbestos material that includes silica sand as its principal filler.